The invention pertains in general to impedance ladder networks or attenuators and, in particular, to a multiplicity of switchably activated resistance elements couPled together to form a network having selectable attenuation settings. Such attenuator networks which have precision graduated settings are commonly known as prescription attenuators.
Prescription attenuators find a variety of uses in the instrumentation field to establish precision voltage references and in the telecommunication field where, for instance, amplifiers are configured with prescription attenuation feedback networks to produce selectable gains for compensating the loss due to various telephone line lengths. In this manner, a single amplifier unit having a plurality of switchable gain settings may be utilized to match any one of a plurality of lines. In addition, if the line characteristics change because of replacement or degradation, a craftsman need only to select a new amplification setting rather than experiment with the gain of the amplifier to arrive at a proper level.
Prescription attenuators used in conjunction with amplifiers typically provide for manually setting the amplifier amplification by way of switches mounted in the amplifier front panel. The switches, traditionally of the double-pole type, connect sections of a constant resistance T-pad between the amplifier input and output to provide amplifier gains in proportion to the attenuation of the activated sections. T-pad attenuators, having three resistive components, are well known in the art. A plurality of T-pads are generally cascaded and the resistive components so chosen so that each T-pad section may be switchably connected into the feedback path. An overall amplifier gain results which is the product (db sum) of the gains attributable to each individually activated section.
The versatility of a prescription gain amplifier is enhanced in view that a craftsman may select any permutation of switch settings to obtain a desired gain. When utilized with T-pad sections, this added versatility is not without the attendant disadvantage of having to extend at least three wires per double-pole switch to the module faceplate or facade where the switches are physically located.
In a typical 24 db gain amplifier having 0.1 db resolution, the number of interconnections for eight switches will normally be twenty-five but may range as high as thirty-two (depending on the number of common connections as will be discussed below).
Recognizing the cost, labor and materials involved, attempts have been made to obviate the multiple inter-connection disadvantage by locating the attenuator components on the faceplate. While this measure alleviates some of the problems, other difficulties, such as trouble shooting defective circuitry, arises out of this attempted solution.
The T-pad network configuration, when used in conjunction with amplifiers to produce prescription gain does exhibit a constant input and output impedance, but such a configuration has the offsetting disadvantage of producing a 6 db gain when all sections are in the nonactivated state. To overcome this aspect, additional loss is inserted either before or after the amplifier stage.
Accordingly, in attenuator networks, it would be highly desirable to substantially reduce the number of inter-connections and the number of attenuator components (and thus cost), yet combine the feature of faceplate-mounted switches wired to the components located on the module board.
In U.S. Pat. No. 4,354,159 a progressive impedance ladder network is disclosed in which a multiple section L-pad attenuator is provided having switchably activated sections. When connected to a source voltage each L-pad section feeds a portion of the voltage to subsequent activated sections thereby providing attenuation. The various section resistive components are selected to produce an ascending inter-section ratio to minimize interference between sections in addition to achieving a desired mixture of individual section attenuations. Importantly, the L-pad sections are individually activated by a single-pole type switch to thereby minimize the number of switches and the switch-to-resistor wire connections.
The attenuator arrangement is utilized as a feedback network with an operational amplifier to provide prescription gain settings. The amplifier output voltage is fed back to its inverting input attenuated by an amount which is dependent upon the attenuator switch settings.
The present invention is an improved and modified version of this progressive network.